ROS: sudo rosdep init出错常规方法都无效后解决办法

问题描述：

sudo rosdep init

安装ros依赖时，终端报错，表明20-default.list无法下载：

ERROR: cannot download default sources list from:
https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/sources.list.d/20-default.list
Website may be down.

1. 自己手动创建 20-default.lis文件

在网上试了些常规方法后，没有解决。无奈之下直接在/etc目录下新建/ros/rosdep/sources.list.d/20-default.list文件（注意sudo rosdep init失败时，/etc下并没有/ros目录，需要依次逐级新建），然后将https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/sources.list.d/20-default.list内容粘贴进去，如下：

# os-specific listings first
yaml https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml osx

# generic
yaml https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/base.yaml
yaml https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/python.yaml
yaml https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/ruby.yaml
gbpdistro https://raw.githubusercontent.com/ros/rosdistro/master/releases/fuerte.yaml fuerte

# newer distributions (Groovy, Hydro, ...) must not be listed anymore, they are being fetched from the rosdistro index.yaml instead

至此，终于解决了20-default.list下载不下来的问题。

注：这里虽然手动新建了20-default.list，但执行sudo rosdep init依然会出现之前的错误，这里我们无视，继续执行下一步。

2.下一步，接下来进行升级，可惜还是报错

rosdep update

终端还是报错：

reading in sources list data from /etc/ros/rosdep/sources.list.d
ERROR: unable to process source [https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml]:
	<urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)> (https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml)
ERROR: unable to process source [https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/base.yaml]:
	<urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)> (https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/base.yaml)
ERROR: unable to process source [https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/python.yaml]:
	<urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)> (https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/python.yaml)
ERROR: unable to process source [https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/ruby.yaml]:
	<urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)> (https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/ruby.yaml)
ERROR: unable to process source [https://raw.githubusercontent.com/ros/rosdistro/master/releases/fuerte.yaml]:
	Failed to download target platform data for gbpdistro:
	<urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)>
Query rosdistro index https://raw.githubusercontent.com/ros/rosdistro/master/index-v4.yaml
ERROR: error loading sources list:
	<urlopen error <urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)> (https://raw.githubusercontent.com/ros/rosdistro/master/index-v4.yaml)>

报的错误原因是python 升级到 2.7.9 之后引入了一个新特性，当使用urllib.urlopen打开一个 https 链接时，会验证一次 SSL 证书，而当目标网站使用的是自签名的证书时就会抛出一个 urllib2.URLError的错误消息。

既然前面尝试了很多办法都没法解决SSL验证问题，那只能想办法在执行rosdep update时尝试定位urllib.urlopen()函数并规避掉SSL验证。

在网上查找后发现确实可以设置SSL验证全局取消(参考：Py 坑之 CERTIFICATE_VERIFY_FAILED)。

因此，在/usr/lib/python2.7/dist-packages/rosdep2/sources_list.py中顶部直接插入两行代码取消SSL验证

import ssl
ssl._create_default_https_context = ssl._create_unverified_context

再次rosdep update，一般到这里即可解决问题。如果还没解决，请往下继续，肯定能解决问题，因为我就是到第3步才解决的

3. nameserver

sudo gedit /etc/resolv.conf

将原有的nameserver 127.0.0.53这一行注释掉，并添加以下两行：

nameserver 8.8.8.8 #google域名服务器

nameserver 8.8.4.4 #google域名服务器

注意：将这两行放在nameserver 127.0.0.53后面，不然百度首页上不去…

到此，问题圆满解决。

参考资料：
ROS:sudo rosdep init出错常规方法都无效后解决办法记录